Slugs - A way of life

Essentially, slugs are nocturnal animals, emerging from their resting places only at night. Activity is closely related to the time of the year, and to atmospheric conditions such as temperature and humidity. Climatic conditions also affect the animal's life cycle - population density, speed of reproduction and growth in particular. Light and available food also make a big difference, while mild winters favour high survival rates and rapid development of egg and young slugs. Slug do not dig through the soil and therefore prefer feeding on the top of the soil where it is easier for them to move and find food.

Most slugs measure between 1 and 15cm. Their bodies can be divided into four parts.

1. The head: it is not defined separately; it can be seen easily owing to the presence of four tentacles. The upper tentacles carry the creature's eyes, and also act as olfactory and tactile organs. They are sensitive enough to allow it to detect food from several feet away. The mouth has two jaws, each lined with small teeth, as well as a tooth-covered tongue, the radula. Together, the number of teeth amount to more than 27,000.

2. The mantle: also called the saddle, it is a clearly distinguished zone just behind the head, which encloses the slug's lung. In some species the
mantle also contains all that is left of the slug's shell - often just a few chalky granules. To the right of the mantle is the slug's respiratory opening - known as the pneumostome.

3. The foot: this is the most muscular part of the slug's body, providing the animal with its means of locomotion. It makes contact with the ground
through the crawling sole. Movement over the ground is eased through secretions of mucus from the caudal mucus gland; these secretions also cover the animal's body and help in keeping it moist and avoiding dehydration.

4. The caudal part: situated behind the mantle and often marked by fissures, it has a more or less complete medio-dorsal keel.

Britain has more than 30 species of slug, more than any other country in the world. Its moderate, damp summers and reasonably warm wet winters mean slugs can be active feeding and breeding for most of the year and maintain a large population. Slugs are not active in low temperatures, but it takes very cold icy conditions to kill them off and they are capable over surviving over winter if the conditions are right.

Slugs are capable of self-fertilisation, but it is rare that a slug will fertilise itself usually embarking on a long and complicated mating ritual. Depending on species slugs can lay anything up to 400 eggs a year. Some lay only once every two years, others annually while the grey slug may lay in five or six batches throughout the year.

If moisture levels are not suitable eggs can remain unhatched for long periods of time until they can receive sufficient moisture.

Depending on species, slugs will live between nine and 18 months, though regional climate may also have an effect on lifespan.

Soil type and availability of food will also influence population size. Heavy soils, or soil not regularly worked retains high level moisture ideal for living and breeding and excess foliage or abundant organic matter gives newly hatched juveniles plenty to feed on.

Oilseed rape is particularly favoured by slugs and susceptible to wide spread damage.

Growers should beware if it has been included in the recent rotation, as slug populations are likely to carry over.

Slug life cycle

There are normally two generations each year for the grey slug (Deroceras reticulatum) and one for the black slug (Arion hortensis). However, more cycles have been observed if the conditions are favourable. Slugs shows a very strong ability to adapt to changing environements.

The grey field slug breds year round exploiting any mild, damp period. Mature adult slugs each lay up to 500 eggs. Typically many eggs and young slugs die from desiccation which keeps populations stable. If weather conditions are paticularly favourable and mortality low, populations may increase to very high levels.

Slugs feed oppotunistically on a wide range of plant material changing from male to female as they mature. They are inactive in unsuitable conditions and resume feeding in mild, damp, still weather.